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Letter
pubs.acs.org/NanoLett
High-Resolution Patterns of Quantum Dots Formed by
Electrohydrodynamic Jet Printing for Light-Emitting Diodes
Bong Hoon Kim,† M. Serdar Onses,‡ Jong Bin Lim,† Sooji Nam,† Nuri Oh,† Hojun Kim,† Ki Jun Yu,†
Jung Woo Lee,† Jae-Hwan Kim,† Seung-Kyun Kang,† Chi Hwan Lee,† Jungyup Lee,† Jae Ho Shin,†
Nam Heon Kim,† Cecilia Leal,† Moonsub Shim,† and John A. Rogers*,†
†
Department of Materials Science and Engineering, Beckman Institute for Advanced Science and Technology, Frederick Seitz
Materials Research Laboratory, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
‡
Departments of Materials Science and Engineering, Nanotechnology Research Center (ERNAM) Erciyes University, Kayseri, 38039,
Turkey
S Supporting Information
*
ABSTRACT: Here we demonstrate materials and operating conditions that allow for high-resolution printing of layers of quantum
dots (QDs) with precise control over thickness and submicron lateral resolution and capabilities for use as active layers of QD
light-emitting diodes (LEDs). The shapes and thicknesses of the QD patterns exhibit systematic dependence on the dimensions of
the printing nozzle and the ink composition in ways that allow nearly arbitrary, systematic control when exploited in a fully
automated printing tool. Homogeneous arrays of patterns of QDs serve as the basis for corresponding arrays of QD LEDs that
exhibit excellent performance. Sequential printing of different types of QDs in a multilayer stack or in an interdigitated geometry
provides strategies for continuous tuning of the effective, overall emission wavelengths of the resulting QD LEDs. This strategy is
useful to efficient, additive use of QDs for wide ranging types of electronic and optoelectronic devices.
KEYWORDS: Electrohydrodynamic jet printing, nanopatterning, quantum dots, light-emitting diode, electroluminescence
C
minimal chemical contamination. Here, we present a highresolution, additive nanofabrication technique that exploits controlled, electrohydrodynamic ejection of fluids through fine
nozzles for the patterned delivery of QDs to a target substrate.29
The method is similar in terms of its additive nature, compatibility
with multiple material “inks”, and programmable definition of
pattern layouts (i.e., maskless operation) to conventional inkjet
techniques, but it offers levels of resolution, registration, and
thickness control that are vastly superior. This electrohydrodynamic jet (e-jet) printing procedure30−32 also does not require
prepatterned topographical or chemical patterns to guide material
flows. Previous work demonstrates compatibility with a wide
variety of inks, ranging from carbon nanotubes (CNTs)33 to
proteins and DNA,34,35 to block copolymers,36,37 conducting
polymers and many others. The work reported here establishes
versatile capabilities when used with solution suspensions of QDs,
olloidal quantum dots (QDs) are nanoscale crystals of
semiconducting materials that can be synthesized and
processed using bulk solution phase techniques.1−7 The sizedependent electrical/optical properties of QDs can be exploited
in unusual classes of electronic and optoelectronic devices with
potential for use in solid-state lighting,8,9 information displays,10−15 imaging detectors, and other systems.16 Quantumdot based light-emitting diodes (QD LEDs) are of particular
interest due to their wide-range color tunability, high brightness, and narrow emission bandwidth.17−19 Challenges remain,
however, in achieving optimized control of charge transport/
light emission and in forming the necessary multilayer device
structures.20,21 Research over the last several years has led
to significant progress on the former set of topics.22 Approaches
to address the latter include conventional ink jet printing,23
advanced techniques of transfer printing12,24,25 and, in initial
feasibility demonstrations, dip pen nanolithography.26−28 The
aim is to enable patterning/stacking of red-green-blue (RGB)
QDs into high-resolution, pixelated geometries with accurate
control of registration, efficient utilization of the materials, and
© 2015 American Chemical Society
Received: October 1, 2014
Revised: December 28, 2014
Published: January 13, 2015
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DOI: 10.1021/nl503779e
Nano Lett. 2015, 15, 969−973
Letter
Nano Letters
A voltage bias applied between a substrate and a metal-coated
glass capillary induces rapid flow of the ink through the fine
opening (e.g., 5 μm) at the end of the nozzle (Figure 1a,b and
Supporting Information 2). The simultaneous programmed
movement of the substrate and control of the voltage enables
patterned delivery of QDs in nearly any geometry. Line patterns
can be efficiently created by operating the e-jet system in a mode
that involves ink delivery in the form of a single, continuous jet,
as shown in Figure 1c,d. In a raster scanning operation with
this mode, QDs can be printed as filled solid patterns (e.g.,
squares of 30 × 30 μm2) as in Figure 1e,f. The spacings between
adjacent printed lines, or the number of overlaid printing
sequences can be varied to control the thicknesses of the patterns
without significant change in the lateral dimensions. Increases
in thickness manifest as increased fluorescence (Figure 1f).
Filled polygons with complex geometries can be designed by
converting an image to numerical commands for automated
printing. Different QDs can be delivered with precise registration
to different regions of a single pattern. Figure 1g,h presents an
example of a printed cartoon image of an apple formed with
green and red QDs. The strong, spatially uniform patterns of
fluorescence suggest full area coverage and uniform thickness
across the pattern. In a pulsatile mode, e-jet printing yields
arrays of circular deposits of QDs with diameters of ∼3.9 μm
(Figure 1i,j), using drop-on-demand operation. Here, control
over the number of delivered droplets determines the thickness
of the printed materials.
Patterning QDs with precise control of their thicknesses and
nanoscale lateral dimensions represent two critical capabilities
for advanced applications. The thickness can be controlled
through a combination of printing parameters including the size
of the nozzle, the stage speed, ink composition, and voltage bias
(Supporting Information 3). As an example, Figure 2a−c shows
printed lines of QDs where different nozzles provide access to
different thicknesses. Here thicknesses evaluated at the center
of the lines and their average widths are 140 nm, 70 nm, 25 nm,
and 1.8 μm, 1.2 μm, 0.25 μm, respectively. The thickness of
a line can be adjusted, without significant change in the width,
through control of the speed of the stage. The dependence of
thickness on stage speed appears in Figure 2d. In addition to
precise control of thickness, e-jet allows patterning with submicron
resolution and large area coverage, as shown for the case of a line
with average width ∼410 nm and length of tens of microns. QDs
form close-packed structures with full area coverage within such
patterns (Figure 2e). The line width variation is typically ∼30 nm
(Figure 2f), due to uncontrolled interactions between the ink and
the substrate during the drying process and by the intrinsic,
particulate nature of the material. Prepatterned topographical and/
or chemical structures on the substrate could further reduce such
variations.
Rectangular/square patterns of QDs represent basic geometries for use in displays. The thickness of the QD layer
critically determines the electro-optical performance of a QD
LED.13,15 Figure 2g and h present AFM images of two different
squares with average thicknesses of 18 ± 10 and 33 ± 7 nm
generated in the continuous jet mode with two different choices
for spacings between adjacent lines during the raster scanning
process. Figure 2i shows a cross-sectional height profile of the
square (roughness ∼7 nm). The roughness typically increases at
thicknesses that correspond to a few layers of QDs. Improvements in this regime may be possible by adjusting the ink compositions (e.g., using mixtures of solvents40) or by postprocessing
of the films (e.g., solvent annealing41).
Figure 1. Electrohydrodynamic jet printing as a route to highresolution patterns of QDs. (a) An optical microscope image of a
metal-coated glass nozzle (5 μm internal diameter at the tip) and a
target substrate during the printing process. (b) Schematic illustration
corresponding to the image of (a). The inks consist of a dilute
solutions (0.25%) of different types of QDs (CdSe/CdZnSeS green or
CdSe/CdS/ZnS red core/shell QDs) in dichlorobenzene. The solvent
evaporates during and immediately after printing. (c,d) QDs printed in
linear geometries: (c) optical and (d) fluorescence images of a pattern
generated in a continuous printing mode. (e,f) QDs printed into filled
square geometries: (e) optical and (f) fluorescence images of an array
of squares (30 × 30 μm2) formed by raster scanning in the continuous
mode. The spacings between adjacent lines decreases from top left to
bottom right. (g,h) QDs printed into complex shapes: (g) optical and
(h) composite fluorescence images of a pattern of red and green QDs
printed in a raster scanning mode to obtain a uniform coverage. (i,j)
QDs printed into arrays of spots: (i) optical and (j) composite
fluorescence images of arrays of red and green QDs printed in a drop
on demand mode. Nozzles with internal diameters of 2 and 5 μm were
used in the results presented above.
through demonstrations of wide-ranging, multicolored patterns
of QDs in various geometries. Such printed structures can be
incorporated into functional QD LEDs with expected properties.
Unusual printed device configurations such as vertical stacks
of QDs enable LEDs with visible light emission at tailored
wavelengths.
Figure 1 demonstrates the ability of e-jet printing to form
diverse patterns of multiple types of QDs with good
registration. Here, solutions of QDs (CdSe/CdZnSeS green
or CdSe/CdS/ZnS red core/shell QDs)38,39 in organic solvents
(dichlorobenzene) serve as the inks (Supporting Information 1).
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Figure 2. Precise control of the sizes and thicknesses of patterns of
QDs formed by e-jet printing. (a−c) AFM images of QDs printed into
lines with different thicknesses. The text over the images indicates the
thickness at the center of the lines. Lines printed using a nozzle with
an internal diameter of (a) 5 μm, (b) 2 μm, and (c) 1 μm. (d) Plot of
the heights at the centers of the lines as a function of the stage speed.
The printing used a nozzle with 1 μm internal diameter. (e) SEM
images of a ∼400 nm wide line pattern printed using a nozzle with
1 μm internal diameter. The images at the right present highmagnification views of the line. (f) Images that highlight the edge
roughness associated with a ∼400 nm wide printed line. (g,h) AFM
images of square pads printed using a nozzle with 2 μm internal diameter.
The thickness of the squares was varied by changing the spacing between
adjacent printed lines. (i) Cross-sectional height profiles obtained from
the AFM images shown in part h.
Figure 3. E-jet printed homogeneous QD array for QD LEDs. (a,b)
Schematic description and photograph of QDs LEDs. (c,d) Optical
micrographs of electroluminescence of green and red QD LEDs. Each
QD pattern was precisely printed using different stage speeds (50 μm/s
for green QDs and 30 μm/s for red QDs, respectively). Plots of current
density−voltage (J−V) and luminance−voltage (L−V) (e,g) and the
external quantum efficiency (EQE) (f,h) of green and red QD LED. A
square pad of QDs with uniform thickness (pixel size: 1 mm × 1 mm)
defined by e-jet printing was used to analyze the device performance.
Figure 3a,b shows schematic illustrations and images of QD
LEDs formed with e-jet printed patterns of QDs. These devices
incorporate (i) indium tin oxide (ITO; on a glass substrate)
as the anode layer, (ii) poly(ethylenedioxythiophene)/
poly(styrenesulfonate) (PEDOT/PSS) as the hole injection
layer (HIL), (iii) poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,40(N-(4-s-butylphenyl)) diphenylamine)] (TFB) as the hole
transport layer (HTL), (iv) CdSe/CdZnSeS green QDs or
CdSe/CdS/ZnS red QDs for emission, (v) ZnO as the electron
transport layer (ETL), and (vi) electron-beam evaporated Al
as the cathode layer.42 Figure 3c,d presents images of the
electroluminescence from e-jet printed green and red QD
LEDs, respectively. The emission properties can be controlled
through the stage speed (50 μm/s and 30 μm/s for green and
red QD LED, respectively) and the distance between the QD
dot patterns. Figure 3e−h shows the current density−voltage
(J−V), luminance−voltage (L−V), and the external quantum
efficiency (EQE) of a typical green and red LED with spatially
uniform layers of QDs. The maximum luminance and EQE
of green QD LED are 36 000 cd/m2 and 2.5%, respectively
(see Supporting Information 4 for current and power efficiency
graph). The results are on par with the best reported performances of QD LEDs of the same device structure fabricated
by spin-casting and/or vacuum deposition techniques.12,43
The maximum luminance and EQE of red QD LED are 11 250
cd/m2 and 2.6%, respectively.
Figure 4a,b provides a schematic illustration of a QD LED
that incorporates a printed array of QDs stacked and patterned
heterogeneously, as the active layer. Type I (Figure 4a) and II
(Figure 4b) devices correspond to “array of lines of green QDs
on a uniform spin-coated film of red QDs” and “array of
alternating dots of green and red QDs”, respectively. All other
layers (anode, HIL, HTL, ETL, and cathode) of these devices
are identical to those of the structure in Figure 3a. Figure 4c−f
presents fluorescence images and photographs of Type I
and Type II devices operating at 4 V. The array of yellow
lines corresponds to the areas of overlap between the green
and red QDs (Figure 4c). Alternating green and red dots
are apparent against the black background as in Figure 4d.
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Figure 5. EL properties of heterogeneously e-jet printed QD LEDs
with type I and II designs. Plots of normalized EL spectra of QD LEDs
with type I (a,b) and II (c,d) designs.
and QD LEDs on a common substrate. This strategy also has
strong potential for integration of QD patterns into functional
semiconducting thin films or with other kinds of printable nanomaterials, for example, CNT, graphene, and DNA. Exploring these
directions as well as furthering the engineering development of the
basic methodologies are topics of current work.
Figure 4. E-jet printed heterogeneous QD array for QD LEDs of type
I and II designation. (a,b) Schematic illustrations of QD LEDs of type
I and II with heterogeneously stacked/patterned QD arrays formed by
e-jet printing, as the active layers. (c,d) Composite fluorescence images
of QD LEDs. In the type I device, the red QD thin film was spin-cast
on the stack of TFB, PEDOT/PSS, and micropatterned ITO
electrode. The line array of green QDs was then formed by e-jet
printing. In the type II device, green and red QDs in the form of arrays
of dot were alternatively printed on the same stack. (e,f) Photographs
of QD LEDs of type I and II designation, operating at 4 V.
■
ASSOCIATED CONTENT
S Supporting Information
*
Details of the quantum dot synthesis, e-jet printer, experimental
conditions for e-jet printing, and current and power efficiency
of e-jet printed green and red QD LEDs are given in this
section. This material is available free of charge via the Internet
at http://pubs.acs.org.
The devices appear red (Figure 4e) and yellow (Figure 4f) to the
unaided eye.
Figure 5a−d shows the normalized electroluminescence (EL)
spectra of QD LEDs with the Type I and Type II configurations. The individual EL emission from each QD layer can be
clearly observed in both systems; however, in type I, the green
emission is suppressed relative to the red (Figure 5a). This
behavior likely arises from some combination of (i) energy
transfer from green to red QDs in this stack, and (ii) higher
electrical resistance, and corresponding lower injected current,
in the green/red stack compared to that of the red film.
The maximum of the normalized green and red EL appears in
the voltage-resolved measurements of Figure 5b. The normalized EL spectra of Type II devices appear in Figure 5c. As
might be expected, here the intensities for green and red light
are comparable (Figure 5d).
This paper demonstrates that advanced techniques in e-jet
printing offer powerful capabilities in patterning QD materials
from solution inks, over large areas. Various homogeneous QD
patterns, for example, dot, line, square, and complex images, are
readily possible, with tunable dimensions and thickness.
Moreover, these arrays as well as those constructed with multiple
different QD materials, directly patterned/stacked by e-jet printing,
can be utilized as photoluminescent and electroluminescent layers.
These capabilities also suggest possibilities in all-QD-based printed
electronic systems that could combine QD transistors,44−48 QD
solar cells,49−54 QD downconverters,55 QD photodetectors,56,57
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: jrogers@illinois.edu.
Author Contributions
B.H.K. and M.S.O contributed equally in this manuscript.
Notes
The authors declare no competing financial interest.
■
■
ACKNOWLEDGMENTS
This material is based on work supported by the Dow Chemical
Company and the Air Force Research Laboratory (AFRL).
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